Python API: implement `PoseBone.children` via `Bone.children`.

Currently `PoseBone.children` is implemented by a linear scan of
the list of armature bones. This is doubly inefficient, since
not only is it scanning all bones, the `obj.data.bones` list
is actually synthetic and generated from Bone children lists.

Instead, use the `Bone.children` native RNA property.

Differential Revision: https://developer.blender.org/D12727

release/scripts/modules/bpy_types.py

@@ -378,10 +378,9 @@ class PoseBone(StructRNA, _GenericBone, metaclass=StructMetaPropGroup):
     def children(self):
         obj = self.id_data
         pbones = obj.pose.bones
-        self_bone = self.bone
 
-        return tuple(pbones[bone.name] for bone in obj.data.bones
-                     if bone.parent == self_bone)
+        # Use Bone.children, which is a native RNA property.
+        return tuple(pbones[bone.name] for bone in self.bone.children)
 
 
 class Bone(StructRNA, _GenericBone, metaclass=StructMetaPropGroup):